<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <style>
       .big{
           width: 500px;
           height: 500px;
           border: 1px solid red;
          /* display: flex;
          align-items: center; */
          /* position: relative; */
           background-color: greenyellow;
           position: relative;
       }
       .small{
           width: 200px;
           height: 200px;
           background-color:skyblue;
           /* position: absolute; */
           /* top: 50%; */
           /* margin-top: -100px; */
           
           /* transform: translateY(-50%); */
           position: absolute;
           top: 50%;
           /* margin-top: -100px; */
           transform: translateY(-50%);

        }
    </style>
    <title>Document</title>
</head>
<body>
    　
        　　　　<div class="big">
        　　　　　　<div class="small"></div>
        　　　　　</div>
        
        　　
</body>
</html>